For many years, pilots have avoided flying through storms and turbulent weather areas. It was common knowledge that many unpredictable and dangerous flight conditions were to be experienced in bad weather.
More recently, there has been mounting concern with the effects upon airborne vehicles resulting from wind-shears and heavy rain conditions. Recent tests by NASA have confirmed that aircraft performance is severely compromised during heavy rains. Unfortunately, very little scientific information is currently available with respect to the effects produced by heavy downpours. Part of this dearth of scientific data is attributable to the lack of instrumentation and methodologies for measuring rain and atmospheric water content during flight.
Simple rain gathering equipment and rain gauge apparatuses for static rain measurement are totally inadequate for this purpose. High speed jet planes have complex aerodynamic surfaces that do not lend themselves to simple rain gathering techniques. Rain generally approaches these surfaces at speeds that present lateral (head-on) droplet introduction to airfoil structures. Liquids flowing over these surfaces form turbulent rivulet streams. These rivulet streams are constantly shifting, making measurement of their substance difficult. At any particular point in time, a surface can be alternately bathed in, or void of, liquid. Surface tension, frictional surface effects, air flow, and a host of other dynamic factors and parameters make the entire measurement problem extremely complex.
It is generally believed that the most dangerous heavy rain condition is presented at low altitudes, and this would obviously effect the aircraft at the worst time, i.e. during landings and take-offs. At these critical times, the airborne vehicle is travelling at approximately 200 miles per hour. Even at these slower speeds, rain conditions are not readily simulated in wind tunnel experiments, because of the difficulty of testing a full scale aircraft in a wind tunnel. Prior studies by the inventors have shown that sub-scale testing is not always accurate because of competing scale effects.
Rainfall rate can be directly correlated to liquid water content (LWC) measurement. High LWC (heavy rain) induced performance degradation is a serious safety hazard. LWC is defined as the mass of liquid water per unit volume of air, and is a measure of rain at altitude.
Although high LWC induced performance degradation is a serious safety hazard, the present state of the art cannot determine the precise period of time, magnitude, and/or at what altitude high LWC presents a dangerous condition. Until this invention, the instrumentation has not been developed to make these tests.
What makes the present invention truly unique, is the fact that the method and system developed to measure LWC is very simple in concept, despite the complexity of the problem. It has been discovered that there is a correlation between LWC and rainfall rate.
The invention described herein eliminates the two major stumbling blocks to measurement of water film thickness, i.e., (1) film instability, and (2) thinness of the film.
As is presently known, air flowing over the water film on the surface of an airborne vehicle causes interfacial instabilities with widely fluctuating film thicknesses. Airfoil surface imperfections and the interfacial instabilities also cause the water film to become discontinuous. Therefore, direct liquid film measurement is all but impossible. Sensors upon a wing of an aircraft, for example, will often experience a dry condition during periods of high LWC.
The invention solves the problem by providing a method and apparatus for building a thick and continuous film layer upon an airfoil surface during flight. The thick and continuous film is sufficiently thick and stable to provide an accurate film measurement by standard sensing devices, such as conductance, capacitive, resistive, optical, and mechanical depth sensors, etc.